1. Field of the Invention
The present invention is concerned with low cost, structurally superior truss assemblies especially designed for use in constructing bridges of from 70 to 200 feet in length, although longer spans are also a possibility. More particularly, it is concerned with such improved truss assemblies wherein the concrete superstructure or deck of the bridge is mechanically composited to carrier truss frames of the overall assembly such that the deck serves as a top chord diaphragm for absorbing live load induced compression and bending forces for the truss framed structures; in this way maximum advantage is taken of the considerable compressive strength and inertial moment of concrete with the least possible dead load to the resultant structure. The assemblies hereof are particularly advantageous inasmuch as they can be factory produced using minimal quantities of structural steel, all of the steel being standard mill stock.
2. Description of the Prior Art
The construction of modern day bridges, such as highway and railroad bridges, is subject to a number of constraints, principally arising from the necessity of adequately distributing and safely absorbing concentrated moving loads imposed thereon without excessive deflection. In the case of highway bridges, the recognition of this problem has led to promulgation of a plethora of rather stringent regulations. For example, one commonly applied code specifies that the completed bridge must be able to sustain, over each possible ten foot travelway within its width, a uniform load of 640 lbs. per lineal foot, and 32,000 lbs. of moving and concentrated load. Additional provisions of the code deal with shear concentrations, and the effects of impact. The bridge must also exhibit sufficient stiffness to strictly limit deflections and oscillations, usually limited to a deflection-to-span ratio of 1 to 800 or less under full live loading with impact. A large number of other provisions also exist for ensuring structural stability due to wind, ice, braking impacts and centrifugal effects.
In addition to the foregoing, a bridge must have a relatively long useful life, require only a minimum of maintenance, and have the ability to withstand climatic freeze/thaw cycles and the effects of deicing compounds used for maintenance of the trafficway.
Thus, although it is entirely possible to demonstrate that a typical bar joist commonly used in building construction to support an office floor or roof can serve as a bridgeway for pedestrians or light vehicular traffic, such a construction is in no way related to the service loading or structural requirements of a modern-day highway bridge.
A number of bridge structures have been developed in the past in attempts to provide safe, stable, yet reasonably priced bridges. One such class of bridges heretofore known are so-called steel truss bridges. There are two basic types of steel truss bridges: (1) where the trussing is above the bridge deck, and the deck roughly conforms to the bottom chord plane of the truss structure; and (2) the underslung truss bridge where the bridge deck is supported by the top chords of the truss structure. Steel truss bridges are seldom used in current bridge construction except in the case of long river spans. This is because of the expense involved in the fabrication and erection of steel truss bridges, and a characteristic depth-to-span ratio of about 1:10 (in order to keep deflection within acceptable limits). Insofar as expense is concerned, for a span of 120-200 feet, the cost of a conventional steel truss bridge would probably be from 70 to 120 dollars per square foot, which is well above certain other types of bridge constructions. Further, the relatively deep trussing required in connection with underslung truss bridges creates problems when maximum clearance under the bridge is required for traffic or high water conditions.
One of the principal elements of cost in connection with known steel truss bridges stems from the fact that much of the truss assembly thereof must normally be constructed in the field using skilled on-site labor and expensive cranes and other equipment. Additionally, such trussing is normally specially designed, necessitates elaborate engineering, and requires large quantities of special order steel, all of which greatly add to the final cost. Finally, additional steel (and hence dead weight and fabrication costs) are normally required because of the need of expensive gusset plates and cross bracing between trusses at the panel points of the trusses.